A basic focal plane shutter comprises two sets or curtains of shutter blades each having a plurality of opaque thin plates pivotally connected to an arrangement of supporting arms. During a film exposing operation, one set of shutter blades is moved from an extended state to a contracted state, whereupon the other set of blades is moved from a contracted state to an extended state. An example of a basic focal plane shutter is disclosed in U.S. Pat. No. 4,024,555 issued May 17, 1977.
A typical mechanically operated focal plane shutter operates by releasing a spring associated with the film drive mechanism which opens the first shutter curtain. Then a second spring is released which closes the second shutter curtain. To re-cock the shutter, the foregoing sequence is not reversed exactly. The first shutter curtain is closed and then the second shutter curtain is opened. This prevents light from exposing the film while the shutter is being re-cocked.
Electromagnetically driven focal plane shutters for cameras have been proposed and patented. Advantages they can have over purely mechanically driven shutters include fewer parts, lower part cost and elimination of a mechanical interface to cock the camera shutter. Examples of electromagnetically driven focal plane shutters are disclosed in U.S. Pat. Nos. 4,338,013 and 4,339,190 issued Jul. 6, 1989 and Jul. 7, 1982, respectively.